1. Field of the Invention
The present invention relates to a step-down circuit that lowers a power-supply potential and controls the lowered potential at a constant internal power-supply potential and also to a semiconductor integrated circuit having this step-down circuit.
2. Description of the Related Art
As shown in FIG. 7, in recent semiconductor integrated circuits, reduction of power consumption and improved reliability of the devices are obtained by supplying a constant internal power-supply potential VII generated by lowering an outside power-supply potential VCC in a step-down circuit 20, to a internal circuit 30. The external power-supply potential VCC may be, for example, 5V and the internal power-supply potential VII may be 3.3 V above the ground potential.
The step-down circuit 20 is provided with a pMOS output transistor 21 which is connected between the external power-supply line SC and the internal power-supply line SI, a differential amplifying circuit 22 for controlling the gate potential VG1 of the output transistor 21 and a reference potential generating circuit 23 for supplying a reference potential VS to be used for comparison against the internal power-supply potential VII to the differential amplifying circuit 22. The differential amplifying circuit 22 is provided with nMOS transistors 41, 42, pMOS transistors 43, 44 and nMOS transistor 45. The nMOS transistor 45 functions as a constant current source with a constant potential VC applied to its gate.
As shown in the FIG. 7, the current that flows through a transistor j is generally designated as I, and the current that flows to the gate of the output transistor 21 as the internal power-supply potential VII fluctuates is designated as IG1, where the equations EQU I45=I41+I42, EQU I43=I41+IG1,
are satisfied and also, since the pMOS transistors 43 and 44 constitute current mirror circuits, EQU I43=IG1,
is satisfied.
When the internal power-supply potential VII is in a stable state, IG1=0, I43=I41, I41=I42, are satisfied.
If the internal power-supply potential VII is lowered from the stable state, the current I42 is decreased by .DELTA. I and the current I41 is increased by .DELTA. I. Therefore, IG1=-.DELTA. A I and, as a result, the potential VG1 of the gate is lowered and the drain current of the output transistor 21 increases to raise the internal power-supply potential VII. In contrast, if the internal power-supply potential VII is raised from the stable state, the current I42 is increased by .DELTA. I and the current I41 is decreased by .DELTA. I. Therefore, IG1 =.DELTA. I and, as a result, the gate potential VG1 is raised and the drain current of the output transistor 21 is decreased to lower the internal power-supply potential VII.
Because the output transistor 21 is required to supply power to the entire general circuit 30, a large driving ability is necessary with 9 gate width of several ten thousands of microns. Because of this, the gate of the output transistor 21 has a large parasitic capacitance and a large driving ability is required of the differential amplifying circuit 22 that controls the gate potential VG1. Since the driving ability of the differential amplifying circuit 22 is in proportion to the size of the current I45, it is necessary that the current I45 be at a large amount.
However, when the semiconductor integrated circuit 10 enters the standby state, the fluctuation of the internal power-supply potential VII becomes relatively small and in that state, therefore, a small driving ability will suffice for the differential amplifying circuit 22. As the current I45 is constant, when the semiconductor integrated circuit 10 is in the standby state, a waste current flows through the differential amplifying circuit 22 which causes the semiconductor integrated circuit 10 to consume excess power.
It may be considered that in order to solve this problem, a step-down circuit for the standby state is added so that separate circuits are provided for the standby state and the active state in such a way that the circuits are switched between each state according to the state of the semiconductor integrated circuit. However, the follow-up of the step-down circuit switching control to respond to the fluctuation of the internal power-supply potential VII is not sufficient, and results in fluctuation of the internal power-supply potential VII that exceeds the allowable value.